the crumb mod.

Info for maintaining and tweaking your amp to perfection.

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the crumb mod.

Post by crumb » Wed Sep 25, 2019 4:21 pm

Don't like the sound of your flyback diodes, the ones you put there to prevent flyback spikes damaging your o/t?
Voltage doubling doubles the spikes, 100 percent mismatching doubles that again, 200 percent mismatch doubles it yet again.
So on a 200 percent mis-match your o/t windings are seeing 4000v.
So on a regular 100w plexi or JMP , jcm800 100 watter ( made after 1966 1/2? to 1981) or any marshall re-issue that uses the 175-0-175 power transformer...just disconnect that centre tap that's running to the centre junction of those first two filter caps, tape it off...and run a 2 watt 220k resistor between the two caps, from+ to -.
In these circuits, voltage is doubled at the centre tap, and because the caps are wired in series, your full H.T appears after the second cap.
You can either remove your flyback diodes, usually attached ( 2 or 3 diodes in series) to pins 3 of your output tube sockets ( 2 of them) or somewhere else where there is a connection to those plate leads, or leave them on.
It actually appears to me as if, rather amazingly, that BOTH Full Wave Bridge, and Full Wave Bridge Voltage Doubling is employed.
The presence of voltage doubling may have been an accident ( the centre tap was simply put there to balance voltage so the filter caps wouldn't be over-rated) , but that doesn't mean the voltage doubling element can't be removed.
Simply perform the mod, and now your amp will have its rectification for H.T created via a simple Full Wave Bridge.
And- enjoy the sound of your amp running out of the 4 ohm jack into an 8 ohm speaker.

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